#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define endl '\n'
const int N = 2e5 + 5;

int n, m;
int a[N];

int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  string s;
  cin >> s;
  n = s.size();
  int l = 0, ans = 0;
  while (l < n) {
    if (s[l] != '0')
      ans++, l++;
    else {
      int h = l + 1;
      while (h < n && s[h] == '0') h++;
      ans += (h - l + 1) / 2;
      l = h;
    }
  }
  cout << ans;
  return 0;
}